To investigate the influence of Azolla (A. filiculoides Lam.) incorporated as a green manure and its subsequent growth as a dual crop with rice on simultaneous methane (CH 4 ) and nitrous oxide (N 2 O) emissions from a flooded alluvial soil planted with rice, a pot experiment with three treatments, chemical fertilizers (NPK) as the control, incorporation of Azolla as green manure (AGM), and AGM plus basal chemical fertilizers (NPK+AGM) was conducted in Tsuruoka, Yamagata, Japan in 2017. AGM and NPK +AGM treatments significantly increased CH 4 emissions at early rice growth stages before 63 days after transplanting (DAT) by 123.0% and 176.7% compared to NPK, respectively. At late rice growth stages (after 63 DAT), only the NPK+AGM treatment significantly increased CH 4 emission by 22.1% compared to NPK. However, percentage of CH 4 emitted after 63 DAT relative to the seasonal CH 4 emission followed the order of NPK (86.2%) > AGM (76.5%) > NPK+AGM (73.3%). Higher CH 4 emissions from AGM and NPK +AGM before 63 DAT were attributed to the incorporated Azolla, while the higher emissions after 63 DAT in all treatment groups were ascribed to rice photosynthesis. AGM and NPK+AGM treatments significantly decreased N 2 O emissions by 71.6% and 81.1% compared to NPK, respectively, at early rice growth stages. Azolla incorporation may have restricted N 2 O emission from initial soil nitrate before 63 DAT and not have contributed to N 2 O emissions after 63 DAT. Significantly higher grain yields were observed under the AGM (32.5%) and NPK+AGM (36.3%) compared to NPK. Together, AGM and NPK+AGM treatments significantly increased seasonal CH 4 emissions by 31.5% and 43.5%, and decreased seasonal N 2 O emissions 3.4-and 4.6-fold compared to NPK, respectively. There were no significant differences in the CH 4 emissions per grain yield among the treatments; however compared to NPK, AGM and NPK +AGM treatments significantly reduced N 2 O emissions per grain yield by 78.7% and 84.1%, respectively.

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Poultry-litter biochar and Azolla as green manure amendments are reported to enhance paddy soil fertility and rice yields. However, whether their co-application in lowland rice paddies has synergistic effects and whether those benefits are accompanied by greenhouse gas (GHG) emissions remains unknown. The objective of this study was to determine the effects of poultry-litter biochar (hereafter: biochar) and its co-application with Azolla as green manure (hereafter: Azolla), on the simultaneous methane (CH 4 ) and nitrous oxide (N 2 O) emissions from a lowland paddy soil planted with rice during a single rice growing season in Tsuruoka, Yamagata, Japan. Biochar and Azolla amendments were applied once before rice was transplanted at a density of 20 t ha −1 and 133.9 kg N ha −1 , respectively. Compared with NPK, NPK + biochar, and Azolla only treatments, Azolla and biochar co-application (i.e., Azolla + biochar) significantly increased CH 4 emissions by 33%–197.6% in the early stages of rice growth (before 63 days after transplanting, DAT), but did not significantly influence CH 4 emissions at both late rice growth stages (after 63 DAT,) and whole rice growth period (112 DAT). Conversely, Azolla + biochar significantly reduced N 2 O emissions by 83.0%–97.1% before 63 DAT, and by 76.4%–95.9% during the whole rice growth period at 112 DAT, with a significantly high interaction between biochar and fertilizer amendments. There were no significant N 2 O emission differences among all treatments after 63 DAT. Additionally, Azolla + biochar significantly increased rice grain yield by 27.3%–75.0%, and consequently, decreased both yield-equivalent CH 4 emissions by 24.7%–25.0% and N 2 O emissions by 81.8%–97.7%. Our findings suggest that the co-application of poultry-litter biochar and Azolla as green manure offers a novel approach to increase rice yield while reducing the emissions of non-carbon dioxide greenhouse gases.

Different weeding frequencies (WFs)―0WF, 2WF, 4WF, 6WF, and 8WF―were adopted in an organic rice field to investigate their influence on weed types, rice and weed biomass, rice tiller and weed number, N concentration, and N uptake. The study field was located in Tsuruoka, Yamagata, Japan. Rice (cv. Sasanishiki) was planted from May to September 2019, and all weeding events were carried out until 49 days after transplantation (DAT) using a rotary weeder. Rice and weed samples were collected at 60, 88, and 110 DAT. Echinochloa crus-galli (E. crus-galli), Schoenoplectus juncoides, and Monochoria vaginalis (Burm. f.) were the most commonly found weeds. High weed diversity was observed at both 6WF and 8WF, E. crus-galli was the dominant weed at 0WF. Compared to 0WF, 2WF displayed a significantly decreased weed biomass. However, at 8WF, the weed biomass was reduced by 79.0%, the rice biomass was increased by 277%, and the rice tiller number was almost three times higher than that at 0WF and 110 DAT. Compared to 0WF, a significantly increased rice N uptake was observed at 4WF, and rice N uptakes achieved at 6WF and 8WF were two and four times higher than those at 0WF, respectively. Higher weeding frequencies provide notable benefits in suppressing weeds and supporting rice growth, especially in N uptake competition with weeds.

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<p>Azolla, a common aquatic fern has been used successfully as a dual crop with lowland rice. It grows rapidly and fixes atmospheric nitrogen for rice paddy. However, its ecological significance especially on greenhouse gases emissions remains unclear. Three independent experiments -two pot (2016 and 2017), and one field in 2019- were conducted to investigate the effects Azolla (<em>A. filiculoides</em> Lam.) either or both as dual cropping and green manure along with rice plant on simultaneous methane (CH<sub>4</sub>) and nitrous oxide (N<sub>2</sub>O) emissions from constantly flooded paddy soil. Under pot setups, dual cropping Azolla as a cover with rice plant significantly decreased seasonal CH<sub>4</sub> emission by 34.7%, with no effect on N<sub>2</sub>O emissions. Suppressed CH<sub>4</sub> emission was likely due to an increase in dissolved oxygen concentration and redox potential at the soil-water interface simulating CH<sub>4</sub> oxidation. However, incorporation of Azolla as green manure into the soil plus dual crop in conjunction with chemical fertilizers significantly increased CH<sub>4</sub> emission by 37.5% but decreased N<sub>2</sub>O emission by 74.5%. The significantly higher CH<sub>4</sub> and lower N<sub>2</sub>O emissions were attributed to the readily decomposable incorporated Azolla, acting both as a source of CH<sub>4</sub> production and N<sub>2</sub>O reduction. Contrary to the pot observations, application of Azolla as a dual crop in conjunction with chemical fertilizer or incorporated as green manure plus dual cropping in the field did not significantly affect seasonal CH<sub>4</sub> emissions, but significantly increased cumulative N<sub>2</sub>O emissions at the middle rice growth stages by 645%—816%, and the total seasonal emission 3.4-fold. The higher N<sub>2</sub>O emissions were partly attributed to large quantities of exogenous organic carbon resulting from the accelerated growth and subsequent senescence of Azolla cover applied as a dual crop as impacted by higher summer air temperatures. Our observations suggest that dual cropping of Azolla with rice has the potential to reduce CH<sub>4</sub> emissions from flooded rice paddies. Conversely, incorporation of Azolla as green manure into the paddy soil plus dual cropping in conjunction with or without chemical fertilizers indicates an inconsistent relationship between CH<sub>4</sub> and N<sub>2</sub>O emissions. Long-term studies are needed to evaluate the relationship between leguminous cover crops and their effects on factors influencing CH<sub>4</sub> and N<sub>2</sub>O emissions from continuously flooded rice paddies.</p>

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